Airborne vector magnetic surveys

ABSTRACT

An aircraft equipped for airborne vector magnetic exploration surveys comprising three magnetometers orthogonally mounted to measure the components of the earth&#39;s vector magnetic field; two rotation sensors mounted to measure the angular orientation of the aircraft; and a recording system to record the measurements of the magnetometers, and rotation sensors. The measured angular orientation is used to orientate the measured components of the earth&#39;s vector magnetic field to derive true vector acro-magnetic (VAM) data from airborne surveys. Also disclosed is a method for processing magnetic data by removing the permanent, induced, and eddy-current magnetic effects of the aircraft from the magnetic data.

TECHNICAL FIELD

This invention concerns an aircraft equipped for airborne vectormagnetic exploration surveys. It also concerns a method of processingvector magnetic data collected during a survey flight of the aircraft.

BACKGROUND ART

Standard airborne magnetic surveys are performed with sensors thatmeasure the total magnetic intensity (TMI) which is the magnitude of thetotal magnetic field vector. The total field is assumed to comprise theearth's field added to a local field dependent on the geology. Surveyareas are sufficiently small that the earth's field may be assumedconstant and so all variations are due to the geology. In practice onesubtracts the magnitude of the earth's field from the measured values toobtain the local field.

Of course, this practice is incorrect because it fails to allow for thefact that the magnetic field is a vector field. The simple subtractionof magnitudes is only correct when the two vectors (earth field andlocal field) are parallel. In general, remanence and anisotropy meanthat parallelism is rarely achieved, however, for local fields that aresmall compared to the earth's field and close to parallel with it, thesimple subtraction is a reasonable approximation.

In situations where the remanent magnetic field is comparable in size tothe earth's field and in a variety of directions, the assumption isunreliable. The breakdown of this assumption will also affect fieldsderived from the TMI such as the reduced-to-pole (RTP) and firstvertical derivative (1VD) fields.

SUMMARY OF THE INVENTION

The invention is an aircraft equipped for airborne vector magneticexploration surveys, comprising:

three magnetometers orthogonally mounted to measure the components ofthe earth's vector magnetic field;

two rotation sensors mounted to measure the angular orientation of theaircraft; and,

a recording system to record the measurements of the magnetometers androtation sensors; where,

the measured angular orientation is used to orientate the measuredcomponents of the earth's vector magnetic field, to derive true vectormagnetic data from airborne surveys, that is vector aero-magnetic (VAM)data.

In such a VAM system, the processing may be done in real time in theaircraft during a survey flight, or after the flight has taken place, inthe laboratory.

The three magnetometers may be flux-gate magnetometers, each measuringthe component of the earth's vector magnetic field along its axis, sothat the triad is able to measure all three orthogonal components.

The rotation sensors may conveniently be provided by an inertialnavigation system, such as may form part of an airborne gravitygradiometer. The sensors may be gyroscopes which measure heading, bankand elevation.

The attitude of the aircraft may be recorded to a precision which shouldallow the magnetic vector components to be corrected to better than 10nT. This compares favourably with uncorrected data, where for instance,in the earth's field of about 60 000 nT, an orientation change of 6degrees can produce a magnetic vector component error of about 10% or 6000 nT.

In a further aspect the invention is a method of processing datacollected during an airborne survey described above, comprising thefollowing steps:

collecting data describing the orientation (attitude) of the aircraftusing one or more rotation sensors (gyroscopes mounted on a gravitygradiometer platform);

collecting vector magnetic field data using a triad of magnetometersorthogonally mounted (flux-gate) in the aircraft; and

using the aircraft attitude data provided by the rotation sensors toorient the magnetometer data; and

then deriving true vector aero-magnetics.

The method involves the rotation of the 3 magnetic field components fromthe aircraft reference frame to the earth's reference frame using aprogram called vectorMagTilt, and a heading correction using a programcalled VectorMagHeadingCorrection. The required parameters for thecorrection are computed using a program called vectorMagCalibrate on thecalibration survey data. The residual noise in the data after theheading correction is still high, but the processed VAM data stillprovides a useful adjunct to the TMI data for mapping and interpretationin areas of strong remanence (for instance, over strongly magnetisedgeology such as banded iron formations).

The method for processing for processing VAM data, may further compriseany one or more of the steps of:

removing the permanent magnet effect of the aircraft from the magneticdata;

removing the induced magnetic effect of the aircraft from the magneticdata; and

removing the eddy-current magnetic effect of the aircraft from themagnetic data.

The formulas for the permanent magnet effect, induced magnetic effectand eddy-current magnetic effect of the aircraft may be based on Leliak(1961)¹.¹ Leliak, P., 1961, Identification and Evaluation of Magnetic-FieldSources of Magnetic Airborne Detector Equipped Aircraft: IRETransactions on Aerospace and Navigational Electronics, Spetember,95-105.

The technique may first involve ignoring the eddy-current effects andsolving for the factors for the permanent magnet and induced magneticdipole fields. The permanent magnet and induced magnetic fields may thenbe computed and removed from the survey data.

The eddy-current factors may then be computed from a high-pass filteredversion of the corrected data. Alternatively, the eddy-current factorsmay be derived line-by-line on the survey data by a regression process.

Furthermore, the data after corrections of permanent magnet, inducedmagnetic and eddy-current effects may go through a residual angle effectcorrection by regression. The final corrected data are then written tothe survey database.

Use of this aspect of the invention provides a significant reduction ofthe noise in the VAM data. Data processing results show excellentperformance of the new technique in noise reduction.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of the invention will now be described with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic diagram of an aircraft equipped for an airbornesurvey.

FIG. 2 is a diagram defining the aircraft-based LTV coordinate system,the world-based NED coordinate system, and the aircraft attitudevariables (heading angle, elevation angle and bank angle).

FIG. 3 is a diagram defining vector magnetic components and vectormagnetic attributes of magnetic field M.

FIG. 4 is a diagram illustrating how the residual magnetic vector iscomputed by subtracting a constant vector from the observed vectormagnetic components. Whilst the inclination and the declination of themagnetic vector is typically confined to a narrow angular range, theresidual magnetic vector typically has inclination and declinationvalues covering the entire angular range.

FIG. 5 is three graphs comparing data from vectorMagHeadingCorrectionand data from vectorMagCorrections on a line of survey data (top: Northcomponent, middle: Down component, and bottom: East component).

FIG. 6 a is a plot of vectorMagResidualIntensity (VMRI) of data fromvectorMagHeadingCorrection; and

FIG. 6 b is a similar plot from vectorMagCorrections.

BEST MODES OF THE INVENTION

The aircraft 10 carries on board an airborne gravity gradiometer (AGG)platform 11, a TMI sensor 12 to measure the total magnetic intensity, atriad of orthogonally mounted flux-gate magnetometers 13 to providevector magnetic field data, and gyroscopes 14 mounted on the AGGplatform 11 to continuously monitor and record the orientation(attitude) of the aircraft. The attitude information is used to controlthe platform and for laser scanner processing and self-gradientcorrections of the AGG data.

The vector magnetic data has three components corresponding to the fieldmagnitude in each of three orthogonal directions. This allows a widevariety of combinations to be formed and mapped. Examples include thecomponents in each of the directions North, East and Down; the magnitudeof the horizontal component; the inclination and declination angles; theTMI and the vector residual magnetic intensity (VRMI). The TMI should bethe same as that measured by the TMI sensor and the difference can betaken as a measure of the vector noise. The VRMI is the magnitude of thevector formed by subtracting the earth's vector magnetic field, forexample as specified by the International Geomagnetic Reference Field(IGRF), from the measured vector field. The VRMI is thus the intensityof the local field and should represent the magnitude of themagnetisation (remanent plus induced) of the local geological sources.

Computer software is used to process VAM data. One computer program,vectorMagTilt, converts the VAM data from an LTV (Longitudinal,Transversal, Vertical) aircraft-based coordinate system to a NED (North,East, Down) world-based coordinate system. Another, vectorMagCalibrate,computes the heading correction coefficients for the NED vector magneticcomponents. Bank and elevation correction may be similarly provided. Thecoefficients are subsequently to be used by a third program,vectorMagHeadingCorrection, to correct the raw NED vector magneticcomponent data for aircraft heading effects, and thence to computerelevant vector magnetic field attributes from the heading-correcteddata, such as horizontal magnetic component H, inclination INC, anddeclination DEC. This program also computes residual magnetic propertiesby subtraction of a constant vector contribution.

The algorithm reads the LTV magnetic components, along with aircraftattitude data (heading-angle, elevation-angle, and bank-angle), andconverts the LTV aircraft-based reading to a NED world coordinatesystem, through the following transformation process:

The vector magnetic flux gate sensors are located in the rear of theaircraft stinger, and record the magnetic field in three orthogonaldirections: L (longitudinal), T (transversal), and V (vertical)

The LTV directions are assumed fixed with respect to the aircraft andare defined as follows:

The LTV directions are orthogonal and form a right-hand coordinatesystem.

The L direction is pointing towards the aft of the aircraft.

The T direction points M degrees upward towards starboard. M is assumedto be 45 degrees.

The V direction points M degrees upward towards port. M is assumed to be45 degrees.

The heading angle H is the aircraft heading in degrees positiveclockwise from North. FIG. 2 depicts a northwesterly heading, andconsequently a heading angle of approximately −45 or +315 degrees.

The elevation angle E is the angle of the aircraft pitch with respect tohorizontal. The elevation angle is defined as positive up and negativedown. FIG. 2 depicts an upward pitch and thus a positive elevationangle.

The bank angle B is the angle of the aircraft roll with respect to thestarboard wing. The bank angle is defined as positive for a bank tostarboard and negative for a bank to port. FIG. 2 depicts a bank toport, and thus a negative bank angle.

The conversion of vector magnetic readings from an aircraft-based LTVcoordinate system to a world-based NED coordinate system is achieved asfollows:

First, the contribution from each of the LTV components to the Ncomponent:

-   The L component projected onto N is:    cos(H+180)·cos(E)·L=−cos(H)·cos(E)·L-   The T component projected onto N is:    cos(H+180)·sin(E)·cos(90−(M−B))·T+sin(H+180)·cos(M−B)·T=−cos(H)·sin(E)·sin(M−B)·T−sin(H)·cos(M−B)·T-   The V component projected onto N is:    cos (H + 180)⋅sin (E) ⋅ cos (M − B) ⋅ V + sin (H + 180) ⋅ cos (90 + (M − B)) ⋅ V = −cos (H) ⋅ sin (E) ⋅ cos (M − B) ⋅ V − sin (H) ⋅ (−sin (M − B)) ⋅ V = −cos (H) ⋅ sin (E) ⋅ cos (M − B) ⋅ V + sin (H) ⋅ sin (M − B) ⋅ V-   Hence the total contribution of the LTV components in the N    direction is:    N=−cos(H)·{cos(E)·L+sin(E)·[sin(M−B)·T+cos(M−B)·V]}+sin(H)·{−cos(M−B)·T+sin(M−B)·V}-   Then, the contribution from each of the LTV components to the E    component:-   The L component projected onto E is:    cos(H+90)l19 cos(E)·L=−sin(H)·cos(E)·L-   The T component projected onto E is:    cos(H+90)·sin(E)·cos(90−(M−B))·T+sin(H+90)·cos(M−B)·T=−sin(H)·sin(E)·sin(M−B)·T+cos(H)·cos(M−B)·T-   The V component projected onto E is:    cos (H + 90)⋅sin (E) ⋅ cos (M − B) ⋅ V + sin (H + 90) ⋅ cos (90 + (M − B)) ⋅ V = −sin (H) ⋅ sin (E) ⋅ cos (M − B) ⋅ V + cos (H) ⋅ (−sin (M − B)) ⋅ V = −sin   (H) ⋅ sin (E) ⋅ cos (M − B) ⋅ V − cos (H) ⋅ sin (M − B) ⋅ V-   Hence the total contribution of the LTV components in the E    direction is:    E=−sin(H)·{cos(E)·L+sin(E)·[sin(M−B)·T+cos(M−B)·V]}+cos(H){cos(M−B)·Tsin(M−B)·V}-   Then, the contribution from each of the LTV components to the D    component:-   The L component projected onto D is:    sin(E)·L-   The T component projected onto D is:    −cos(E)·sin(M−B)·T-   The V component projected onto D is:    −cos(E)·cos(M−B)·V-   Hence the total contribution of the LTV components in the D    direction is:    D=sin(E)·L−cos(E)·[sin(M−B)·T+cos(M−B)·V]

This example is based on the assumption that the LTV coordinate systemis perfectly aligned with the aircraft coordinate system. That is, thatthe L-axis aligns perfectly with the aircraft longitudinal axis, and notwith the stinger longitudinal axis. (The stinger is mounted with aslight positive pitch with respect to aircraft axis). In practice, therewill often be a small angular displacement between the LTV coordinatesystem defined by the three fluxgates and the aircraft coordinate systemreferenced by the heading, elevation and bank angles. The vectorMagTiltprogram therefore includes offset angles to correct for this angulardisplacement. The offset angles will vary between aircraft andvectorMagTilt allows for their adjustment as required.

vectorMagCalibrate is used on vector magnetic calibration flights, whichare performed at the start of each AGG campaign. The calibration flightconsists of eight flight lines flown at high altitude (preferably morethan 3000 ft above the ground). The lines are all flown at the samealtitude. The lines are flown in the eight headings 0°, 45°, 90°, 135°,180°, 225°, 270°, and 315°. The lines should each be at least 3 km longand they should all intersect at the same point, roughly at the halfwaymark for each line. The survey essentially forms a star or a pizza with4 pair-wise parallel flight lines (for example at headings 0° and 180°,45° and 225°, 90° and 270°, 135° and 315°). The heading correctioncoefficients are output to screen at the end of the program execution.

The program first determines all the intersections of the calibrationlines. Once all the intersections have been determined the algorithmdetermines the average position of the intersections, and outputs thestatistics on how well the pilots managed have all the calibration linesintersect at one central point.

Having determined a central intersection point the program now extractsthe attitude (heading-, elevation-, and bank-angle) and vector magneticcomponents (NED) from the database at the central intersection point foreach of the calibration line.

The extracted data may be used to verify the heading-angle dependency ofthe uncorrected NED data or how well the subsequent sine-functionfitting has performed.

The algorithm now attempts to fit a scaled sine function of the headingangle to each of the NED components. The functions to fit are:N _(OBS) ≈C _(N,1)·sin(head_angle−C _(N,2))+C _(N,3)E _(OBS) ≈C _(E,1)·sin(head_angle−C _(E,2))+C _(E,3)D _(OBS) C _(D,1)·sin(head_angle−C _(D,2))+C _(D,3)

-   Note that currently no corrections are being applied for bank- and    elevation-angle effects.

Having established the coefficients C_(N,1), C_(N,2), C_(N,3), etc., wecan at a later stage perform the heading correction as:N _(OBS,corr) ≈N _(OBS) −C _(N,1)·sin(head_angle−C _(N,2))E _(OBS,corr) ≈E _(OBS) −C _(E,1)·sin(head_angle−C _(E,2))D _(OBS,corr) ≈D _(OBS) −C _(D,1)·sin(head_angle−C _(D,2))

Once the algorithm has computed the correction coefficients theestimated main magnetic field strength, inclination and declination areoutput for checking purposes. The estimated main magnetic fieldinclination and declination values will usually be within 3 degrees ofthe associated IGRF values for the calibration site location.

Having determined the correction coefficients the algorithm displaysthese on screen. The data is presented in a format that is appropriatefor cut-and-paste insertion into the parameter file forvectorMagHeadingCorrection.

This example does not attempt to incorporate bank-angle orelevation-angle into the correction model. It only uses attitude andvector magnetic information from the central intersection point, as itis assumed that the magnetic value should remain unchanged over thisgiven point irrespective of the aircraft heading.

vectorMagleadingCorrection is used to Process VAM Data.

The correction coefficients to be applied in the heading correctionprocess are those computed and output by the program vectorMagCalibrate.

The heading correction is achieved by subtracting a scaled,phase-shifted sine function of the heading angle from the individual NEDvector magnetic components:N _(OBS,corr) ≈N _(OBS) −C _(N,1)·sin(H−C _(N,2))E _(OBS,corr) ≈E _(OBS) −C _(E,1)·sin(H−C _(E,2))D _(OBS,corr) ≈D _(OBS) −C _(D,1)·sin(H−C _(D,2))

The correction coefficients C_(N,1), C_(N,2), C_(E,1), etc. are outputby the program vectorMagCalibrate to screen, and must be specified inthe parameter file for vectorMagHeadingCorrection The screen output fromvectorMagCalibrate is presented in a format that is appropriate forcut-and-paste insertion into the parameter file forvectorMagHeadingCorrection.

The algorithm reads the raw NED vector magnetic component data, alongwith aircraft attitude data (heading-angle, elevation-angle, andbank-angle), and corrects the raw NED vector magnetic component data foraircraft heading effects by subtracting the scaled and phase-shiftedsine-functions (above).

Having completed the heading correction, vectorMagHeadingCorrectioncomputes relevant vector magnetic field attributes from theheading-corrected data, such as horizontal magnetic component H,inclination INC, and declination DEC.

FIG. 3 depicts the various vector magnetic components and attributesassociated with a magnetic field M. From FIG. 3 we get that thehorizontal magnetic vector component H is computed as:H={square root}{square root over (N ² +E ²)}.The magnetic inclination INC is computed as;${INC} = {{\tan^{- 1}\left( \frac{D}{H} \right)}.}$The magnetic declination DEC is computed as:${DEC} = {{\tan^{- 1}\left( \frac{E}{N} \right)}.}$

In addition to the “standard” vector magnetic attributesvectorMagHeadingCorrection also computes the residual magneticattributes by first completing a subtraction of a constant vectorcontribution from the heading corrected NED vector magnetic components.The option exists to subtract either the survey-wide averages of the NEDvector magnetic components, or to subtract the NED vector magneticcomponents derived from the vector magnetic calibration flight.

Surveys have been flown over a variety of formations. A comparison ofthe total magnetic intensity (TMI) data with the intensity of theresidual vector magnetic (VRMI) data showed very similar results forweakly remanent formation but significantly different results for morestrongly remanent formations. This demonstrated that the vector magneticresults are able to provide improved data for prospecting.

There are also a variety of effects which cause varying magnetic fieldsfrom the aircraft itself In particular, ferro-magnetic parts of theaircraft will have a magnetic field induced from the earth's main fieldwhich will change as the orientation of those parts varies relative tothe earth's field; electrical conductors will have eddy currentsgenerated leading to the production of secondary fields; and remanentlymagnetised parts of the aircraft, producing constant magnetic fieldcomponents in the aircraft-based LTV coordinate system, will generatechanging magnetic fields in the NED coordinate system as the aircraftchanges orientation.

These particular effects which depend on aircraft orientation can bewritten as functions of the orientation angles heading, bank andelevation provided by our rotation sensors. In practice, poor knowledgeof the physical properties of each relevant aircraft part, limitedknowledge of their position and motion and the high complexity of thetotal system may make this impractical. However, it is possible to uselinear regression of the measured VAM data against the angular variablesto estimate the coefficients of the linear terms of these functions;vector aero-magnetic compensation.

The main steps are:

a) identify the key regressors using data collected on a calibrationflight;

b) estimate the regressor coefficients (the sensitivity of eachcomponent to each regressor) by standard linear regression; and

c) correct survey VAM data by subtraction of the effects calculated asthe product of each coefficient against each regressor.

An additional technique is based on the principle of removing thepermanent magnet effect, induced magnetic effect and eddy-currentmagnetic effect of the aircraft from the magnetic data. This techniquehas been implemented in processing software using two computer programs,a modified vectorMagCalibrate program and a new codevectorMagCorrections. Only a single program vectorMagCorrections needsto be run to process vector magnetic data on a survey. Prior toprocessing vector magnetic data, the processing parameters will need tobe computed by vectorMagCalibrate on calibration survey data.

The derivation of formulas for removing the permanent magnet effect,induced magnetic effect and eddy-current magnetic effect of the aircraftis based on the model of Leliak (1961) and is given below

The measured magnetic field M is composed of the earth's field El(including ore-body effect), the permanent magnet field of the aircraftA, the induced magnetic field of the aircraft I, and the eddy-currentmagnetic field E, HenceM=H+A+I+E

In the aircraft reference frame, these are three equations at eachobservation point for the three magnetic field components,M _(L) =H _(L) +A _(L) +I _(L) +E _(L)  (1)M _(T) =H _(T) +A _(T) +I _(T) +E _(T)  (2)M _(V) =H _(V) +A _(V) +I _(V) +E _(V)  (3)

At the interception point of the eight calibration lines, the earth'smain field is known (the IGRF field or that calculated invectorMagCalibrate) in the earth's NED reference frame. Thus, the LTVcomponents H_(L), H_(T), and H_(V) can be calculated by rotation withthe known aircraft attitude information

The permanent magnet field components A_(L), A_(T), and A_(V) areconstants that are independent of aircraft attitude.

The L component of the induced magnetic field of the aircraft at thesensor isI _(L) =H _(L) LL+H _(T) TL+H _(V) VL  (4)where LL is the magnetic field in L direction due to induced magneticdipoles in the L direction for an unit inducing field, TL is themagnetic field in L direction due to induced magnetic dipoles in the Tdirection for an unit inducing field, and VL is the magnetic field in Ldirection due to induced magnetic dipoles in the V direction for an unitinducing field.

Similarly, the T component of the induced magnetic field of the aircraftat the sensor isI _(T) =H _(L) LT+H _(T) TT+H _(V) VT  (5)and the V component of the induced magnetic field of the aircraft at thesensor isI _(V) =H _(L) LV+H _(T) TV+H _(V) VV  (6)

Here, (LL, TL, VL, LT, TT, VT, LV, TV, VV) are only dependent on thedimension, shape, and susceptibility of the parts of the aircraft body,but independent of the orientation of the aircraft.

The eddy-current magnetic field is produced by eddy currents in theaircraft body. A change of magnetic flux through a conducting loop willgenerate a current proportional to the tine derivative of the flux inthe loop. This current will produce a secondary magnetic field opposingthe change in the magnetic flux. As the aircraft hull effectivelyconsists of conducting loops of aluminium, these loops will experience achange in magnetic flux as the aircraft changes direction in the earth'smagnetic field. These current loops will generate a secondary magneticfield measurable as the eddy-current field at the sensor. The Lcomponent of the eddy-current field can be written as $\begin{matrix}{E_{L} = {{\frac{\partial H_{L}}{\partial t} \cdot {ll}} + {\frac{\partial H_{T}}{\partial t} \cdot {tl}} + {\frac{\partial H_{v}}{\partial t} \cdot {vl}}}} & (7)\end{matrix}$where ll is the magnetic field in L direction due to eddy-currentmagnetic dipoles in the L direction for an unit inducing field, ti isthe magnetic field in L direction due to eddy-current magnetic dipolesin the T direction for an unit inducing field, and VL is the magneticfield in L direction due to eddy-current magnetic dipoles in the Vdirection for an unit inducing field. Similarly, $\begin{matrix}{E_{T} = {{\frac{\partial H_{L}}{\partial t} \cdot {lt}} + {\frac{\partial H_{T}}{\partial t} \cdot {tt}} + {\frac{\partial H_{V}}{\partial t} \cdot {vt}}}} & (8) \\{E_{V} = {{\frac{\partial H_{L}}{\partial t} \cdot {lv}} + {{\frac{\partial H_{T}}{\partial t} \cdot {tv}}{\frac{\partial H_{V}}{\partial t} \cdot {vv}}}}} & (9)\end{matrix}$

Here, (ll, tl, vl, lt, tt, vt, lv, tv, vv) are only dependent on thedimension, shape, and electrical conductivities of the parts of theaircraft body forming the conductive loops, but independent of theorientation of the aircraft.

Substituting equations (4)-(9) into equations (1), (2) and (3), weobtain $\begin{matrix}{{H_{L} + A_{L} + {H_{L} \cdot {LL}} + {H_{T} \cdot {TL}} + {H_{V} \cdot {VL}} + {\frac{\partial H_{L}}{\partial t} \cdot {ll}} + {\frac{\partial H_{T}}{\partial t} \cdot {tl}} + {\frac{\partial H_{V}}{\partial t} \cdot {vl}}} = M_{L}} & (10) \\{{H_{T} + A_{T} + {H_{L} \cdot {LT}} + {H_{T} \cdot {TT}} + {H_{V} \cdot {VT}} + {\frac{\partial H_{L}}{\partial t} \cdot {lt}} + {\frac{\partial H_{T}}{\partial t} \cdot {tt}} + {\frac{\partial H_{V}}{\partial t} \cdot {vt}}} = M_{T}} & (11) \\{{H_{V} + A_{V} + {H_{L} \cdot {LV}} + {H_{T} \cdot {TV}} + {H_{V} \cdot {VV}} + {\frac{\partial H_{L}}{\partial t} \cdot {lv}} + {\frac{\partial H_{T}}{\partial t} \cdot {tv}} + {\frac{\partial H_{V}}{\partial t} \cdot {vv}}} = M_{V}} & (12)\end{matrix}$

Using Leliak's (1961) model as encapsulated in equations (10)-(12), wecan solve for the 24 unknowns (A_(L), A_(T), A_(V)), (LL, TL, VL, LT,TT, VT, LV, TV, VV), and (ll, tl, vl, lt, tt, vt, lv, tv, vv) from thecalibration survey data as follows. At the intersection point of thecalibration lines, we can set up the equations (10)-(12) for each line.Since there are eight calibration lines, we have a total of 24equations. In theory we should be able to solve for the 24 unknowns fromthese 24 equations. However, since the magnitude of eddy-currentmagnetic field is much smaller than the permanent magnetic and inducedmagnetic fields, direct solutions of equations (10)-12) do not yieldgood solutions for (ll, tl, vl, lt, tt, vt, lv, tv, vv). In practice, wefirst ignore the eddy-current terms and solve for the 12 (A_(L), A_(T),A_(V)) and (LL, TL, VL, LT, TT, VT, LV, TV, VV) factors for thepermanent magnet and induced magnetic dipoles. The permanent magnet andinduced magnetic fields are then computed and removed from thecalibration survey data. The eddy-current factors are computed fromhigh-pass filtered versions of the corrected data All the 24 factors arecomputed from a modified version of the vectorMagCalibrate program.VectorMagCalibrate also output corrected data to the calibration surveydatabase.

All these factors are input parameters to the vectorMagCorrectionsprogram. In the current implementation of vectorMagCorrections, theeddy-current factors derived by vectorMagCalibrate are not used.Instead, new eddy-current factors are derived line-by-line on the surveydata by a regression process. Furthermore, the data after corrections ofpermanent magnet, induced magnetic and eddy-current effects go through aresidual angle effect correction by regression. The final corrected dataare then written to the survey database.

FIG. 5 shows a comparison of the data from vectorMagHeadingCorrectionand the data from vectorMagCorrections on a line of survey data. Avisual inspection suggests a noise reduction improvement of a factorbetween 3 to 10. The improvement using the new technique for vectormagnetic data processing is obvious.

FIG. 6 a and 6 b shows a comparison of thevectorMagneticResidualIntensity (VMRI) of data fromvectorMagHeadingCorrection and vectorMagCorrections. The VMRI is themagnitude of the residual magnetic vector after subtracting the vectorIGRF earth field from the data. The improvement using the new techniquefor vector magnetic data processing is obvious as shown in FIG. 6 a and6 b.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the invention as shown inthe specific embodiments without departing from the spirit or scope ofthe invention as broadly described. The present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive.

1. An aircraft equipped for airborne vector magnetic explorationsurveys, comprising: three magnetometers orthogonally mounted to measurethe components of the earth's vector magnetic field; two rotationsensors mounted to measure the angular orientation of the aircraft; and,a recording system to record the measurements of the magnetometers androtation sensors; where, the measured angular orientation is used toorientate the measured components of the earth's vector magnetic field,to derive true vector magnetic data from airborne surveys, that isvector aero-magnetic (VAM) data.
 2. An aircraft according to claim 1,where the three magnetometers are flux-gate magnetometers, eachmeasuring the component of the earth's vector magnetic field along itsaxis, so that the triad is able to measure all three orthogonalcomponents.
 3. An aircraft according to claim 1, where the rotationsensors are provided by an inertial navigation system that forms part ofan airborne gravity gradiometer.
 4. An aircraft according to any one ofclaims 1, where the rotation sensors are gyroscopes which measureheading, bank and elevation.
 5. A method of processing data collectedduring an airborne survey claimed in any preceding claim, comprising thefollowing steps: collecting data describing the orientation (attitude)of the aircraft using one or more rotation sensors; collecting vectormagnetic field data using the triad of magnetometers orthogonallymounted in the aircraft; and using the aircraft attitude data providedby the rotation sensors to orient the magnetometer data; then, derivingtrue vector aero-magnetic data.
 6. A method according to claim 5 wherethe processing is done in real time in the aircraft during a surveyflight.
 7. A method according to claim 5 where the processing is doneafter the flight has taken place.
 8. A method according to claim 5,where the processing involves the rotation of 3 LTV components in theaircraft reference frame to earth's NED reference frame, and a headingcorrection is applied.
 9. A method according to claim 8, whereparameters for the correction are computed on calibration survey data.10. A method according to claim 5, including the step of removing thepermanent magnet effect of the aircraft from the magnetic data.
 11. Amethod according to claim 5, including the step of removing the inducedmagnetic effect of the aircraft from the magnetic data.
 12. A methodaccording to claim 5, including the step of removing the eddy-currentmagnetic effect of the aircraft from the magnetic data.
 13. A methodaccording to claim 5, where the formulas for the magnet effects arebased on Leliak (1961).
 14. A method according to claim 13, where thesteps first involve ignoring the eddy-current terms and solving for thefactors for the permanent magnet and induced magnetic dipoles.
 15. Amethod according to claim 13, where the permanent magnet and inducedmagnetic fields are computed and removed from the calibration surveydata.
 16. A method according to claim 13, where the eddy-current factorsare then computed from a high-pass filtered version of the correcteddata.
 17. A method according to claim 13, where the eddy-current factorsare derived line-by-line on the survey data by a regression process. 18.A method according to claim 13, where the data, after corrections ofpermanent magnet, induced magnetic and eddy-current effects, go througha residual angle effect correction by regression.
 19. A method accordingto claim 13, where the final corrected data are then written to thesurvey database.